1. Field of the Invention
The present invention relates to a magnetoresistive effect element for reading magnetic field intensity of a magnetic recording medium and the like as a signal, a thin film magnetic head that is provided with the magnetoresistive effect element, and a head gimbal assembly and a magnetic disk device that contain the thin film magnetic head.
2. Description of the Related Art
In recent years, along with the advancement of high recording density of a hard disk drive (HDD), further improvement is demanded in the performance of a thin film magnetic head. As a thin film magnetic head, a composite type thin film magnetic head is widely used having a structure in which a reproducing head having a read-only magnetoresistive effect element (MR element) and a recording head having a write-only induction-type magnetic transducer element are laminated.
Currently, as a reproducing head, a magnetoresistive effect element of a so-called CIP (Current In Plane) structure (CIP-GMR element) that is referred to as a spin-valve GMR element and that is operated by flowing a current in parallel to a film surface of the element is widely used. In a reproducing head, the spin-valve GMR element of such a structure is positioned between an upper and a lower shield layers formed with soft magnetic metal films and is arranged in a form of being sandwiched from above and below by insulation layers that are referred to as gap layers. A recording density in a bit direction is determined by a gap (reproduce gap length) between the upper and lower shield layers.
Along with the increase of the recording density, there is an increasing demand for a narrower shield gap and a narrower track with respect to the reproducing element of the reproducing head. Due to a narrower track of the reproducing element and a reduction in height of the element that accompanies the narrower track, an area of the element decreases. There is a problem that heat dissipation efficiency decreases as the area decreases in a conventional structure and thus an operation current is limited from a point of view of reliability.
In order to solve such a problem, a GMR element of a CPP (current perpendicular to plane) structure (CPP-GMR element) is proposed in which an upper and a lower shield layers (an upper shield layer and a lower shield layer) and an MR element are electrically connected in series and an intershield insulation layer is not needed. This is an essential technology for achieving a recording density that exceeds 200 Gbits/in2.
Such a CPP-GMR element has a lamination structure containing a first ferromagnetic layer and a second ferromagnetic layer that are formed in a manner sandwiching a conductive nonmagnetic intermediate layer from both sides. The lamination structure of a typical spin-valve type CPP-GMR element is a lamination structure in which, from a substrate side, a lower electrode, an antiferromagnetic layer, a lower ferromagnetic layer, a conductive nonmagnetic intermediate layer, an upper ferromagnetic layer and an upper electrode are sequentially laminated.
A magnetization direction of the lower ferromagnetic layer, which is one of the ferromagnetic layers, is pinned in such a manner that, when an externally applied magnetic field is zero, it is perpendicular to a magnetization direction of the upper ferromagnetic layer. The magnetization direction of the lower ferromagnetic layer is pinned by having an antiferromagnetic layer adjacent to the lower ferromagnetic layer to impart unidirectional anisotropy energy (which is also referred to as an “exchange bias” or a “coupling magnetic field”) in the lower ferromagnetic layer via an exchange coupling between the antiferromagnetic layer and the lower ferromagnetic layer. For this reason, the lower ferromagnetic layer is also referred to as a magnetization pinned layer. On the other hand, the upper ferromagnetic layer is also referred to as a free layer. Further, it is also proposed that the magnetization pinned layer (the lower ferromagnetic layer) have a three-layer structure of a ferromagnetic layer, a nonmagnetic metal layer and a ferromagnetic layer (so-called “laminated ferrimagnetic structure” or “synthetic pinned structure”). This allows a strong exchange coupling to be imparted between the two ferromagnetic layers of the magnetization pinned layer (the lower ferromagnetic layer) and the exchange-coupling force from the antiferromagnetic layer to be effectively increased, and in addition, this allows influence of a static magnetic field that is generated from the magnetization pinned layer on the free layer to be reduced. Therefore, the “synthetic pinned structure” is currently widely used.
However, in order to meet the demand for ultra-high recording density in recent years, further thinning of the MR element is required. In such circumstances, a new GMR element structure is proposed that has a simple three-layer lamination structure of a ferromagnetic layer, a nonmagnetic intermediate layer and a ferromagnetic layer as a basic structure as disclosed, for example, in U.S. Pat. No. 7,019,371B2, U.S. Pat. No. 7,035,062B1, and the like. In this GMR element structure, as illustrated in FIG. 17, two ferromagnetic layers 61, 62 are exchange-coupled in such a manner that the magnetizations 61a, 62a of the ferromagnetic layers 61, 62 are mutually antiparallel. A permanent magnet HM is arranged at a back-region position that is opposite to an air bearing surface (ABS) that corresponds to a medium-opposing surface of the element. An initial state is created by a bias magnetic field generated from the permanent magnet HM, in which the magnetizations 61a, 62a of the two ferromagnetic layers 61, 62 are respectively inclined about 45 degrees with respect to a track width direction and are substantially orthogonal to each other (see FIG. 18). When the element in this initial magnetization state detects a signal magnetic field from the medium, magnetization directions of the two ferromagnetic layers 61, 62 change in a way like that when a pair of scissors cuts a piece of paper. As a result, a resistance value of the element changes. For convenience, such an element structure is referred to as a DFL (Dual Free Layer) element structure in the present specification.
When the DFL element structure is applied to a TMR element or a CPP-GMR element, as compared to a common spin-valve type CPP-GMR element, a “read gap” that is a gap between the upper and lower shield layers 61, 62 can be significantly narrowed. Specifically, the antiferromagnetic layer that is needed for a common spin-valve type CPP-GMR element is no longer needed and, in addition, the ferromagnetic layer in the “synthetic pinned structure” is also no longer needed.
In order to form the DFL element structure in the conventional technology, it is necessary that the two ferromagnetic layers 61, 62 are exchange-coupled in such a manner that the magnetizations 61a, 62a of the two ferromagnetic layers 61, 62 are mutually antiparallel. Such a structure can be easily formed by inserting a noble metal such as Au, Ag, Cu, Ir, Rh, Ru or Cr between the two ferromagnetic layers 61, 62 to generate an exchange coupling between the two ferromagnetic layers 61, 62.
However, a disadvantage may occur in a TMR element, since an insulating film such as an aluminum oxide (AlOx) film or a magnesium oxide (MgO) film must be interposed between the two ferromagnetic layers in order to obtain a tunneling effect, it is difficult to generate a strong exchange coupling between the two ferromagnetic layers. As a result, it is extremely difficult to have the magnetizations of the two ferromagnetic layers in an antiparallel state.
Further, in a head structure using the above-described DFL element structure in the conventional technology, in order to generate sufficient bias magnetic field intensity from the permanent magnet HM such as CoPt arranged at the back-region position that is opposite to the ABS to form the initial state, it is necessary to increase the thickness of the permanent magnet HM. Increasing the thickness of the permanent magnet HM means that a merit that the DFL element structure is a structure in which the read gap can be narrowed cannot be fully enjoyed. To increase the thickness of the permanent magnet HM and narrow the read gap will reduce gaps between the permanent magnet HM and an upper and a lower shield layers 71, 72 and a problem may occur that the bias magnetic field generated from the permanent magnet HM leaks to the upper and the lower shield layers 71, 72 so that the application of the bias magnetic field to the element becomes insufficient, and the resistance change of the element cannot be sufficiently detected.
Further, in the head structure using the above-described the DFL element structure in the conventional technology, the permanent magnet HM is arranged at the back-region position that is opposite to the ABS, and the above-described initial state in the two ferromagnetic layers 61, 62 is formed by applying the bias magnetic field from the permanent magnet HM to the two ferromagnetic layers 61, 62. However, a problem may also occur that there is a risk that the bias magnetic field from the permanent magnet HM may leak from the element, and due to the leakage magnetic field, a signal may be erroneously written to a medium, or a signal recorded in a medium may be demagnetized or degaussed.